The present invention relates to a cold cathode fluorescent tube which enlarges an effective light emitting region and a liquid crystal display device using the cold cathode fluorescent tube, and more particularly to a cold cathode fluorescent tube which can be suitably used in a lighting device of a liquid crystal display device and a liquid crystal display device using the cold cathode fluorescent tube.
Among various lighting devices, a discharge tube has been popularly used as a light source which exhibits low power consumption, high luminance or has a small size. With respect to this discharge tube, a low-pressure discharge tube which seals an inert gas and mercury in the inside of an outer sheath tube made of a transparent insulation material such as glass which applies phosphor on an inner surface thereof has been well known as a fluorescent lamp. This type of low-pressure discharge tube is classified into a hot cathode type discharge tube which uses hot electrons and a cold cathode type discharge tube which uses cold electrons.
For example, as the light source of a lighting device of the liquid crystal display device, a cold cathode fluorescent lamp (CFL) which discharges cold electrons and excites the phosphor to emit light has been adopted. In general, an electrode which discharges cold electrons is made of a metal material such as nickel or the like. Since the electrode has the sputtering characteristics, the electrode is dissipated and is consumed. Accordingly, the electrode of the cold cathode fluorescent tube is required to ensure a certain degree of size. On the other hand, when the size of the electrode is increased, a surface area of the electrode is also increased and hence, the current density per unit are is lowered whereby a sputter quantity of the electrode material on an inner wall of the outer sheath tube at the time of discharging (particularly at the time of aging before shipping a product) is decreased.
In this type of cold cathode fluorescent tube, the shortening of the discharge start time (discharge starting time) is requested. Particularly, in a pitch dark, there has been a drawback that a delay is generated with respect to time for starting the discharge. There has been reported an advantageous effect that when a portion of the electrode which is made of a nickel material in general is sputtered on the inner wall of the outer sheath tube, the electrons induced from the sputter film shorten the discharge start time. In making use of a cup-shaped electrode, by setting an outer diameter of the electrode (particularly an outer diameter of an opening portion) smaller than an inner diameter of the outer sheath tube, sputtering of the electrode material onto the inner wall of the outer sheath tube is promoted. However, sputtering of the electrode material makes the electrode per se wear and hence, the lifetime of the electrode is shortened. Further, even when the sputtering film is formed using an aging step, the shortening of the discharge start time achieved by the aging step is not sufficient.
To decrease this delay of the discharge start time, it is effective to place a material which induces the discharge in the inside of the outer sheath tube (particularly at a portion close to the electrode). Accordingly, various methods including following methods have been adopted conventionally:
(1) A metal compound having high electron emission performance, for example, cesium compound such as cesium chromate is adhered to a surface of the electrode.
(2) Using a mixture of the above-mentioned metal compound having high electron emission performance such as the cesium compound and a mercury emission material, the metal compound having high electron emission performance such as the cesium compound or the like is dispersed in the inside of the tube at the time of heating the mercury for emission in manufacturing steps.
(3) The electrode is heated by supplying a high current (for example, approximately 8 to 15 mA when the outer diameter of the electrode is 1.7 mm) to the electrode thus dissipating a portion of the electrode material to form a sputter film on the inner wall of the outer sheath tube in the vicinity of the electrode.
(4) Alumina is added to the inside of a phosphor film.
As the specific means to decrease the delay of the discharge start time, a sputter film which contains metal or a metal compound as a main component is formed on the inner wall of the outer sheath tube in the vicinity of the cup-shaped electrode thus inducing the discharge due to cold electrons which are generated by a voltage applied to the electrode, whereby the discharge start time is shortened. Further, to increase an area of the electrode, the electrode is formed into a cup shape, a thin film containing the above-mentioned metal material is formed on an inner wall surface thereof, and the thin film containing the metal material is evaporated by heating to form the sputter film on the inner wall of the outer sheath tube in the vicinity of the electrode in an aging step. Here, with respect to this type of prior art, for example, Japanese Patent Laid-Open No. 76617/2001 (literature 1), Japanese Patent Laid-Open No. 231133/2002 (literature 2) and the like are named.